This invention relates to the conversion of alkenes, especially lower alkenes and specifically ethylene, to carbonyl derivatives such as acetaldehyde (when the alkene is ethylene) and ketones (when the alkene is propylene or a higher alkene) by a two-stage process in which the alkene is reacted (in the first stage of the process) with an aqueous acidic solution of an oxidant catalyst system comprising a noble metal (especially palladium) together with a redox agent comprising cupric chloride, the carbonyl compound then being separated from the spent catalyst solution which is then (in the second stage of the process) reoxidized with a source of molecular oxygen (typically air) before being recycled to the first stage of the process for the conversion of additional quantities of the alkene.
Although the conversion of ethylene to acetaldehyde is presently the most important variant of the process commercially, it can also be employed to convert other alkenes, particularly lower alkenes and more particularly alkenes having up to about six carbon atoms, to corresponding ketone derivatives. For example, propylene can be converted to acetone and 1-butene and 2-butene can be converted to methyl ethyl ketone.
The process is described, for example, in a paper by Dr. J. Smidt in "Chemistry and Industry" (Jan. 13, 1962), pages 54-61. This paper describes both the two-stage process, with which the present invention is concerned, and a closely-related single-stage process, which does not employ a separate catalyst reoxidation step and which is less closely related to the present invention.
The details of the reaction mechanisms are not pertinent to the present invention, with one exception: this is that the formation of chlorinated by-products, which are continuously withdrawn from the reaction product mixture in the course of product recovery in purification, results in a continuous depletion of the chloride content of the catalyst solution, which in turn results in reduced catalyst activity (i.e. reduced productivity per unit volume of the alkene-oxidation reactor), necessitating replenishment of the chloride content of the catalyst solution by adding hydrochloric acid. This makeup acid, in aqueous solution, is customarily admixed with a slip stream drawn from the main stream of oxidized catalyst solution leaving the catalyst reoxidation stage of the process and being forwarded to the alkene-oxidation stage. This slip stream, admixed with the makeup hydrochloric acid, is passed through a "catalyst regeneration" zone in which, in addition to incorporating the fresh acid into the solution, the heated mixture of catalyst solution and hydrochloric acid is also allowed to react at elevated temperature (e.g. about 160.degree.C) to decompose certain undesirable organic byproducts including oxalates. The process can, if desired, be operated without a separate catalyst regeneration zone, in which case the makeup hydrochloric acid is simply incorporated into the depleted catalyst solution entering the catalyst reoxidation stage of the process.
It is known that the composition of the catalyst solution should be controlled within certain limits for optimal chemical efficiency, but heretofore it has been necessary as a practical matter to allow more fluctuation in its composition than is desired. More particularly, the nature of the catalyst solution is such that it does not readily lend itself to rapid "on-line" chemical analyses from which fine adjustment of its composition can be readily and quickly carried out in the production plant. On-line analysers for either chloride or copper ion in this solution are not available. Because of this, the ordinary mode of operation has heretofore entailed periodic conventional chemical analysis followed by a rather extended adjustment of the hydrochloric acid flow on what amounts to a trial and error basis in an effort to compensate for the continuing loss of chloride moiety from the system and maintain a constant catalyst composition.
The method of hydrochloric acid flow adjustment just described has two deficiencies. First, to the extent that it results in under-treatment, it results in under-utilization of the alkene-oxidation reactor. Such under-utilization, by which is meant in the present context operation at roughly 90% or less of maximum reactor capacity, is more than ordinarily important in this particular process, since the highly corrosive nature of the catalyst solution requires use of process equipment which is fabricated of, or lined with, titanium, so that, more than in many other chemical processes, under-utilization of the reaction apparatus is a substantial economic drawback. Second, over-treatment, as distinguished from under-treatment, has consequences which are also unacceptable. Specifically, over-treatment also results in a lowering of catalytic activity to such an extent as to cause a rapid "upset" in the reaction with a sudden decrease in conversion and resulting rise in the amount of alkene which passes through the reactor unconverted.
Heretofore, plant operation of the alkene oxidation process has been characterized by periodic "upsets" caused by operating with either too much or too little hydrochloric acid, such that, even with the most conscientious attention, the productivity of the reaction system falls short of its maximum capability by an amount which is of the order of 10% or more.
It is an object of the present invention to provide an improved process control system by means of which the reactor productivity can be easily maintained at its optimal level in processes of the sort described wherein an alkene is oxidized to a carbonyl derivative with an aqueous solution of a noble metal, particularly palladium, and a redox agent, particularly copper chloride, in a two-stage system of tubular reactors the first of which is an alkene-oxidation reactor while the second is a catalyst solution reoxidation reactor in which the components of the catalyst solution are reoxidized with a source of molecular oxygen, such as air.
It is a specific object to provide a reliable and effective method for controlling the rate of addition of hydrochloric acid into a catalyst regeneration zone operated in conjunction with the catalyst reoxidation system contained in a two-stage process, as described above, for oxidizing a lower alkene such as ethylene to a carbonyl derivative such as acetaldehyde.
Other objects will be apparent from the following detailed description, example, and claims.